Method for producing a tool which can be used to create surface structures in the sub-mum range

ABSTRACT

The invention relates to a method for producing a tool, and a tool which can be used to create optically active surface structures in the sub-μm range. Said tool comprises a support surface to which surface structures are applied by depositing material, said surface structures being raised in relation to the support surface. The invention is characterised in that the support surface is in-directly bonded with a mask ( 2 ) in which openings ( 3 ) having diameters in the sub-μm range are provided or can be placed; the support surface and the mask are subjected to a coating process during which coating material is deposited onto the support surface via the openings ( 3 ) in the mask; the mask is removed from the support surface when a partial amount of the average height of the end structure pertaining to the surface structures is obtained; and the coating process is continued without a mask and with similar or different coating material.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a method for producing a toolwhich can be used to create optically active surface structures in thesub-μm range, having a support surface onto which relief surfacestructures are applied by means of the deposition of materials.Furthermore, a corresponding tool is described.

[0003] 2. Decription of the Prior Art

[0004] In many light-optical applications, reflections of light have adisturbing respectively energy-diminishing effect on technical andoptical surfaces and interfaces. For example, the readability ofdisplays is distinctly impaired by the reflection of external lightsources. Furthermore, in the case of glazed solar cell modules,reflection losses occur on the cover thereby considerably reducing thesolar cell's efficiency.

[0005] To decrease reflection respectively increase transmission,technically complicated single layer or multi-layer systems havehitherto been applied to the surfaces in order to antireflect them.These surface finishes are, however, not only expensive but alsounsatisfactory, because their effect depends largely on the angle ofincidence and the wavelength of the light impinging on the surfaces.

[0006] Most recent antireflecting methods pursue a different approach,notably to design the transition in the refractive index at the surfaceof each optical component and not as hitherto abruptly but rathercontinuously. In order to do this, each surface is “roughened”respectively structured in order to copy the moth-eye effect known fromnature. In designing such type surface structures, the average lateraldimensions of each structure must be distinctly smaller than thewavelength of the incident light in order to prevent the light fromscattering and the increased diffuse reflection accompanying it.Furthermore, for an effective reduction of reflection, it is necessarythat the structures have an aspect relationship, i.e. the relationshipbetween the vertical to the lateral dimensions of the surfacestructures, in the magnitude of 1 or higher.

[0007] The production of such type surface structures with suitablyshaped tool surfaces using hot stamping or injection molding processesby, for example, heating an optical element for surface coating by meansof hot stamping until above the glass temperature of the material ofwhich it is composed and structuring it at its surface accordingly witha stamping tool bearing a negative image of the microstructuring. Suchtypes of stamping tools with a microstructured stamp can be made ofnickel using galvanic methods, in which periodic surface microstructurescan be obtained using photolithographic techniques. Such types ofperiodic surface structures are very efficient in their optical effectand far superior to the prior art stochastically distributedmicrostructure surfaces. They, however, have the drawback that they areproduced with nickel. However, such types of stamps, also called“master”, possess low high-temperature resistance due to the materialthey are made of, and can therefore only be utilized for shaping thesurface of organic glasses, such as for example optical elements made ofPMMA. Moreover, the lifetime of such nickel master tools is short asdistinct wear can be observed with repeated use due to their material.Finally, the production method of periodic structures usingphotolithography has the disadvantage that open surfaces or asphericalsurfaces, often needed in technology, cannot be structured.

[0008] Steel mill tools coated with a layer of mechanically resistant,high-temperature resistant, ceramic material are prior art for theaforedescribed tools for transferring periodic structures ontopreferably optical surfaces. With the suited selection of layerdeposition parameters, these layers fulfill basically the aforesetrequirements with regard to the aspect relationship and the lateraldimensions of the microstructures but not with regard to the regular,periodic arrangement of the surface structures as is the case with theaforementioned nickel master stamps.

[0009] Due to the non-periodicity of the microstructures applicable inthe layers of mechanically resistant ceramic materials, these surfacestructures are called stochastic surface structures. In these stochasticsurface structures not only the lateral arrangement of the singleelements of the structure in the mechanically resistant ceramic materialare irregular, but the quite varied growth of the single elements of thestructure can be observed. The presence of large structural elementswithin the microstructure surface, however, leads to an undesiderableincrease in diffuse reflection and at the same time to a decrease indirect reflection when being formed. With reference to this see: A.Gombert, W. Glaubitt, K. Rose, J. Dreibholz, B. Bläisi, A. Heinzel, D.Sporn, W. Döll, V. Wittwer, “Subwavelength-Structured AntireflectiveSurfaces on Glass”, Thin Solid Films 351 (1999), No.1-2, pp. 73-78.

[0010] The article “Fabrication of a Micro-Patterned Diamond Film bySite-Selective Plasma Chemical Vapor Deposition”, by Sakamoto et al.,Thin Solid Films, Elsevier-Sequoia S. A. Lausanne, CH, vol. 334, No.1-2,4 December 1998 (1998-12-04), pp. 161-164, ISSN:0040-6090, describes amethod of selective coating of substrates with a diamond film. Suchtypes of substrates provided with diamond film structures are used inpromising applications in technology, such as for example inmicroelectronics for field effect transistors. In order to obtainselective diamond depositions with dimensions of the structures in thesub-gm range, a property of diamond is utilized that it deposits on aplatinum layer with approximately a 10⁴ greater affinity than, forexample, on a SiO₂ surface. If suited platinum structures are providedon a substrate surface, all that is needed is full-surface PVD coatingof such a type of pre-structured SiO₂ substrate with diamond, which onlydeposits significantly on the Pt-structured areas. The selective,diamond coated substrate yielded by this method meets microelectronicneeds, but it cannot be used as a heat-resistant tool, in the sensedescribed in the introduction, due to the purposely selective coating.

SUMMARY OF THE INVENTION

[0011] The object of the present invention is to further develop amethod for producing a tool and to provide said tool which can beutilized for fabricating optically active surface structures in thesub-μm range and which has a support surface on which relief surfacestructures are applied over the support structure by means of materialdeposition that, on the one hand, prevents the drawbacks arising withthe aforedescribed stochastic surface structures, i.e. undesirable highdiffuse and direct reflection, and, on the other hand, nonethelessensures utilizing the tool under high temperatures in order to be ableto antireflect optical or technical surfaces made of not only organicmaterials but also of inorganic materials with it.

[0012] The solution to this object on which the present invention isbased is set forth in claim 1 describing the invented method. Thesubject matter of claim 12 is an invented tool which like the inventedmethod is further developed by the features of the subclaims. Moreover,the advantageous features that further improve the inventive idea aregiven in the description with reference to the accompanying drawings.

[0013] According to the present invention, a method for producing a toolis disclosed, which can be used to create optically active surfacestructures in the sub-μm range and which has a support surface formed orprocessed to a desired macroscopic contour on which relief surfacestructures are applied by means of material deposition over the supportsurface, is further developed in such a manner that the support surfaceis directly contacted with a mask which is provided or can be providedwith openings with diameters in the sub-μm range. Then the supportsurface with the mask is subjected to a coating process in which thecoating material is also deposited through the openings of the mask ontothe support surface. Upon reaching a partial amount of an average heightof an end structure of the desired surface structures, the mask isremoved from the support structure and the coating procedure is thencontinued without the mask. The partial surface structures formeddirectly after removal of the mask are used as growth or crystallizationseeds for the continued coating procedure by means of which materialdeposition and further crystal growth occurs preferably at thecrystallization seeds but also between the surface structures. Thuscoating material is deposited on the entire support surface.

[0014] A particularly favorable time to remove the mask from the supportsurface is reached when the developing elements of the surfacestructures have reached approximately 0.5% to 80% of their desired endheight.

[0015] By dividing the coating procedure into two parts, single surfacestructure elements are obtained whose lateral structure dimensionsdiminish with increasing height of the structure due to the subsequent,preferred material deposition on the coating seeds. In this manner,column-like formations with diminishing diameters are formed as surfacestructures on the support surface. In this manner protrusions can becompletely avoided so that transfer of the surface structure ontooptical or technical surfaces is readily possible in a moldingprocedure.

[0016] The arrangement with which the individual surface structureelements are deposited on the support surface is determined by thearrangement of the openings in the mask. In the simplest case, a layerof conventional photoresist varnish applied uniformly onto the supportsurface serves as the mask material. With the aid of suitedphotolithographic exposure techniques, openings of any desired size anddistribution can be exposed into the photosensitive varnish layer independence on the exposure patterns.

[0017] Another possible way to produce the desired mask provides for theuse of an aqueous suspension in which the colloidal, bead-shapedparticles are dispersed and which are applied to the support surface.After a drying process, the colloidal particles remaining on the supportsurface form the mask layer with the wide gaps between the particlescorresponding to the openings through which material deposition formingthe aforedescribed surface elements can occur. Further details forproducing such a mask layer are given in the following sections of thedescription with reference to the accompanying drawings.

[0018] Especially suited as materials for forming the surface structureare the following elements: Ti, Al, Zr, Cr, Mo, Ni, W, Si B, and C aswell as mixtures of elements of the just listed examples of elements.Furthermore, at least one of the layer-forming elements may be at leastpartially oxidized or nitrated.

[0019] Suited as materials for developing crystallization seeds in thefirst coating step are, in addition to the aforementioned elements,particularly also Ag, Au, Pd, Pt.

[0020] With the aid of such a coating, it is possible to produce ahigh-temperature resistant layer which can withstand even temperaturesup to over 850° C. and permits molding of the surface structure onoptical or technical surfaces made not only of organic, preferably,light-transparent plastics but, in particular, also composed ofinorganic materials, such as glass.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The invention is made more apparent in the following usingpreferred embodiments with reference to the accompanying drawings by wayof example without the intention of limiting the scope or spirit of theoverall inventive idea.

[0022]FIGS. 1a to d show a schematic representation for producingsurface structures on a support surface,

[0023]FIG. 2 shows a representation of surface structure elements on thesupport surface, and

[0024]FIG. 3 shows a cross section of a coated support surface bearingsurface structure elements,

[0025]FIGS. 4a to c show the production of a mask of colloidal particlesand their use as a mask for applying coating seeds.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

[0026] The drawings of FIGS. 1a to d show on the left a lateral view andon the right a top view of the respective steps of the method. FIG. 1adepicts a lateral and a top view of the support of a tool, with asupport surface 1 which is to be selectively coated. In FIG. 1b, a mask2, which is provided with a multiplicity of openings 3 where the supportsurface 1 is surrounded free of the material of mask 2, is placeddirectly on the support surface 1. The mask material 2 can, for example,comprise a photoresist, which is removed locally and selectively at thesites of openings 3 after corresponding exposure and the subsequentetching process. Depending on the manner of exposure, the openings 3form in a specific configuration within the mask 2. A top view of FIG.1b shows an example of opening geometry and a correspondingconfiguration. The openings 3 represent dark areas on the supportsurface, the remaining areas are covered by the mask material 2, herewhite areas.

[0027] The mask 2 provided with openings 3 is then coated in asubsequent coating procedure in which particularly high-temperatureresistant materials are deposited, preferably with the aid of a PVDprocess. In FIG. 1c, the mask 2 and the openings 3 are covered by acoating material 4, which deposits directly onto the support surface 1particularly inside the openings 3. The coating of support surface 1 andthe mask 2 can be interrupted as soon as the height of the coatinginside the openings 3 reaches a level corresponding to approximately0.5% to 80% of the average projected end structure height of theindividual elements of the surface structure. When such a layerthickness is reached, mask 2 and the layer 4 are removed from supportsurface 1 with the aid of selective etching techniques. The materialdeposits 5 remain as single surface structure elements which havedeposited directly on the support surface inside the openings 3, as FIG.1d shows both in a lateral view and a top view. Then the coating processis continued and the surface structure elements 5 remaining on thesupport surface 1 after removal of the mask serve as coating seeds asshown in FIG. 2. In the further course of the coating process, thecoating material is deposited on the entire support surface butdeposited with preferred growth of the single structures in the regionof the seeds.

[0028] The surface structures created with the aid of the aforedescribedmethod usually have, depending on the openings 3 inside the mask 2, 6 to400 surface structure elements per μm². Moreover, the single elements ofthe structure are usually spaced between 50 and 400 nm apart.

[0029] Especially good properties can be obtained if the surfacestructure has single surface structure elements whose respective sizeand shape can be divided into at least two different groups of surfacestructure elements. Fundamentally, the structure elements of one groupare larger than those of the other group. Furthermore, the lateralspaces of the centers of the single structure elements are distributedapproximately at least two characteristic, average distances M1 and M2.The following relationships have proven particularly advantageous forthese two average distances: the value M1 should lie in a region between50 nm and 180 nm, whereas the average distance M₂ has values between 200nm and 400 nm. Especially good results are yielded, for example, ifM₁=170 nm and M₂=300 nm. Furthermore, it was discovered that therelationship of M₂ and M₁ should preferably be greater than 1.1 andsmaller than 8, in particular 1.72.

[0030]FIG. 3 shows a cross section of a tool with a support surface 1,which can preferably be created out of a metallic material, inparticular out of a steel or out of ceramic materials or out of silicon.A multiplicity of surface structure elements 5, which are shapedcolumn-like and taper upward are deposited onto the support surface 1.Provided between the single surface structure elements 5 are materialdeposits also in different coating heights due to the continued coatingprocedure. The surface structure elements preferably have heights whosehighest heights h.H. in relation to the respective surrounding lowestlayer height differs by the factor 1.6 from the lowest surface structureelements n.H. in the layer. Furthermore, the single surface structureelements 5 preferably have structure heights H lying between 50 and 1000nm.

[0031]FIG. 4 shows an alternative masking and coating method to the onedescribed in FIG. 1. In this case, the masking of the support structure1 occurs before the coating process by means of applying an aqueoussuspension of colloidal particles 6. The suspension 6 is applied ontosupport surface 1 with the aid of conventional microtitter devices 7.The a drying process according to FIG. 4b occurs resulting in a denselypacked two-dimensional arrangement of particles on the support surface.This is followed by the coating procedure 8 required for placingcrystallization seeds. The colloidal particles 9 remaining on thesupport surface 1 cover the support surface in such a manner that gapsform between the immediately adjacent colloidal particles 9corresponding to the aforedescribed openings 3. The size of theparticles 9 determines the spacing as well as the size of the openings3. Preferably, the particles 9 can be composed of polymer materials orinorganic materials, such as for example silicon oxide and usually havea diameter of 50 nm to 50 μm. FIG. 4c shows a coating pattern yieldedafter the colloidal particles have been coated and correspondinglyremoved from the support surface 1.

[0032] The colloidal particles 9 can be selected in such a manner thattheir sizes are distributed by an average value (with a monomode sizedistribution being reached) or by at least two concrete values (with abimode distribution being reached).

[0033] The invented method permits producing novel tools for surfacestructuring technical respectively optical surfaces. These tools are, inparticular, high-temperature resistant. Furthermore, the applied new,submicrostructured mechanically resistant layers, preferably on steel,have the following advantages: the suited selection of the materialto-be-applied onto the support surface permits obtaining ahigh-temperature resistance that allows using such molding tools notonly for organic glasses but also for inorganic glasses, for examplesilicate glass.

[0034] The coating procedure is not restricted to plane supportsurfaces. With the aid of the invented method practically any desiredcontour can be coated permitting, in particular producingantireflective, non-plane optical components, such as for exampleaspheres.

[0035] A special variant of the tool designed according to the inventionprovides structuring the support surface before it is subjected to thecoating procedure in the aforedescribed manner. In this case, thesupport surface is provided with indentations partially or full-surface.At the surface, the diameter of these indentations lies in the rangebetween 1 μm and 30 μm. The indentations reach with approximately thesame dimensions into the interior of the support substrate in such amanner that an aspect relationship of approximately 1 is present. Theindentations are evenly distributed over the support surface and bordereach other directly with their on- the-support-surface-lying peripheraledges.

[0036] If the preceding measures are met, based on the invented method,after fabrication, a tool is yielded with which technical surfaces canbe produced which possess dirt-repelling and or oil-repellingproperties. List of Reference Numbers 1 support surface 2 mask, maskmaterial 3 openings 4 coating material 5 surfaces structure elements 6colloidal suspension 7 pipette device 8 ion respectively particle flowin the PVD process 9 colloidal particles

What is claimed is:
 1. A method for producing a tool which can be usedto create optically active surface structures in the sub-gm range,having a support surface onto which relief surface structures areapplied over said support surface by means of material deposition,wherein said support surface is directly contacted with a mask in whichopenings with diameters in the sub-μm range are provided or can beprovided, said support surface including said mask is subjected to acoating process in which the coating material deposits through saidopenings of said mask onto said support surface, and said mask isremoved from said support surface when a partial amount of an averageend structure height of said surface structures is reached and thecoating procedure is then continued without said mask using the samecoating materials or different coatings materials, which are depositedonto said entire support surface in such a manner that said coatingmaterial settles between said surface structures.
 2. The methodaccording to claim 1, wherein said removal of said mask is carried outwhen approximately 0.5% to 80% of the average end structure height ofsaid surface structures has been reached.
 3. The method according toclaim 1 or 2, wherein said coating is carried out in such a manner thatsurface structures form whose lateral structure dimensions decrease withincreasing distance from said support surface.
 4. The method accordingto one of the claims 1 to 3, wherein, in order to produce said openingsinside said mask, said support surface is covered with a mask layer andsaid mask is perforated using photolithographic, electron beam or ionbeam methods as well as a selective material removal method.
 5. Themethod according to claim 4, wherein the mask material is a photoresistlayer or a photosensitive coating.
 6. The method according to one of theclaims 1 to 3, wherein a suspension with colloidal, bead-shapedparticles which form said mask on said support surface after a dryingprocess is applied onto said support surface.
 7. The method according toclaim 6, wherein said colloidal particles are composed of polymermaterials or of an inorganic material and have a diameter in the 50 nmto 50 μm size range.
 8. The method according to one of the claims 1 to7, wherein said mask is designed in such a manner that an arealdistribution of said openings of 6 to 400 openings per 1 μm² isprovided.
 9. The method according to one of the claims 1 to 8, whereinsaid openings are spaced approximately between 50 and 400 nm apart. 10.The method according to one of the claims 1 to 9, wherein said coatingprocess is a PVD process in which at least one of the following elementsis used as the coating material: Ti, Al, Zr, Cr, Mo, Ni, W, Si, B, C.11. The method according to claim 10, wherein for coating said mask andthe directly accessible support surface, alternatively or in additionthe elements Ag, Au, Pd, Pt are utilized for developing crystallizationseeds.
 12. The method according to claim 10 or 11, wherein at least oneof said elements utilized as said coating material in the continuedcoating process is at least partially oxidized or nitrated.
 13. A toolfor surface structuring and fabrication of optically active surfacestructures in the sub-μm range, produced according to a method accordingto claims 1 to 12, having a support surface onto which relief surfacestructures are applied by means of material deposition over said supportsurface, wherein applied onto said support surface, which is designed,plane, or not plane, in particular curved, is a layer ofhigh-temperature resistant material having relief surface structuresover said support surface, said surface structures having dimensions inthe sub-gm range having heights between 50 and 1000 nm and havingdistances apart between 50 and 400 nm.
 14. The tool according to claim13, wherein said surface structures have practically the identical shapeand size or possess discrete shapes and sizes.
 15. The tool according toclaim 14, wherein said surface structures are dividable into at leasttwo discrete shape and size groups the arrangement of which is subjectto an order on said support surface.
 16. The tool according to claim 15,wherein the centers of said surface structures each have mutual lateralspacing distributed by at least two characteristic, average distances M₁and M₂, with for M₁ and M₂ apply: M₁=M₂ or M₂>M₁, with 50 nm<M₁<180 nmand 200 nm<M₂<400 nm or with M₁=170 nm and M₂=300 nm or with 1.1<M₂/M₁<8or with M₂/M₁=1.72.
 17. The tool according to one of the claims 13 to16, wherein the magnitude of the greatest difference in height betweenthe highest surface structure and the respective surrounding indentationis less than a factor 1.6 of the magnitude of the smallest difference inheight between the smallest surface structure and the respectiveindentation surrounding it.
 18. The tool according to one of the claims13 to 17, wherein the surface structures have different structuralheights and elevations measured from the respective indentationssurrounding them, which lie between 50 and 1000 nm.
 19. The toolaccording to one of the claims 13 to 18, wherein said support surface ismade of metal, preferably steel or stainless steel, of ceramicmaterials, quartz glass or of silicon.
 20. The tool according to one ofthe claims 13 to 19, wherein the uncoated support surface is smooth orstructured.
 21. The tool according to claim 20, wherein said structureduncoated support surface has indentations the diameter of which at saidsupport surface and depth of which lies in the range from 1 μm to 30 μm.22. The tool according to claim 21, wherein said indentations haveaspect relationships which lie in the order of magnitude of
 1. 23. Useof said tool according to one of the claims 13 to 22 as a stamping meansfor surface structuring optical or technical surfaces in order to obtainantireflective surfaces.
 24. Use according to claim 23, wherein saidtool can be utilized at temperatures above 400° C.
 25. Use according toclaim 23 or 24, wherein said to-be-antireflective optical or technicalsurfaces being composed of organic, preferably light-transparentplastics, or inorganic materials, such as glass.
 26. Use according toclaim 23, wherein a tool whose support surface is prestructuredaccording to claims 21 or 22 is utilized for fabricating dirt- repellingand/or oil-repelling technical surfaces.